Waste heat refers to heat generated by such furnaces as distillation columns, incinerators, blast columns, and smelting columns used in various industrial fields for which no useful application is found. Waste heat produced by furnaces, boilers, dry facilities, and air conditioning facilities is often discharged in the form of combustion flue gas, whereas waste heat produced during heating, distillation, evaporation, purification, and separation processes is cooled off or condensed, and then released as cooling water. Waste heat at relatively high temperatures can be utilized for the production of low-pressure steam or for preheating boiler feed water. On the other hand, waste heat at relatively low temperatures is merely discharged to the environment due to technical difficulties as well as high equipment costs for its recovery.
Particularly, a petrochemical process may be divided into two general steps of (1) a reaction step and (2) a separation/purification step. From the perspective of energy, such a process may also be divided into (1) a heating step wherein heat is supplied, e.g., to the lower part of a column for reaction or separation/purification and (2) a cooling step wherein the upper part of a column is cooled off. In general, the heating step uses steam as a heat source, and the cooling step uses cooling water as a cooling source.
In a petrochemical process, steam is used to reboil the lower part of a column, resulting in a hot fluid. The hot fluid in the upper part of the column is subjected to cooling by cooling water or a fan cooler, which is then recycled to the column as a reflux or produce a distillate. FIG. 1 is a schematic view of a conventional distillation column wherein a hot fluid generated in the upper part of the column is cooled by cooling water. With reference to FIG. 1, the distillation column 100 comprises a column 101, a steam reboiler 102 for supplying heat to the column wherein the steam reboiler is connected to a lower part of the column, and a cooling device 103 for cooling a hot fluid discharged from an upper part of the column by cooling water wherein the cooling device is connected to the upper part of the column. The column 101 receives heat from the steam reboiler 102, and the hot fluid discharged from the upper part of the column 101 is cooled in the cooling device 103 and subsequently recycled to the column 101.
The low-temperature heat source discharged from a column overhead may be utilized by, e.g., a mechanical vapor recompressor (MVR) or a thermal vapor recompressor (TVR), instead of being cooled by cooling water.
An MVR uses a mechanical compressor for compressing vapor discharged from a column overhead to raise the temperature and the pressure thereof. The vapor can then be reused as a heat source for reboiling the column or for generating low-pressure steam. However, an MVR requires high investments and operation costs. Further, the column may not be safely operated in case the MVR system fails since no cooling source is then available.
In a TVR, high pressure vapor is used as a driving force to compress low pressure waste steam, the compressed steam being reused as a heat source. The investment and operation costs for a TVR are relatively low. However, it requires a large amount of high pressure vapor for producing usable steam, which may result in a loss of secondary steam due to the disruption of the overall steam balance.
Accordingly, there has been a need for a system for recovering waste heat generated in the upper part of a column to thereby improve the energy efficiency thereof.